const s1 = Symbol('s1')

console.log(s1 === s1[Symbol.toPrimitive]()) //=> true

// obj怎么改写，可以输出20
// const obj = {}
const obj = {
  x: 10,
  [Symbol.toPrimitive]() {
    return this.x
  }
}

console.log(10 + obj) //=> 20
